Mohamed Barj

Excitation of N2(B 3Pig) in the nitrogen short-lived afterglow

The excitation of the N2(B 3g) state in microwave discharges ignited in flowing nitrogen and in their short-lived afterglow is investigated. Simultaneous measurements in the post-discharge on this species and the N2+(B 2u+) ion through emissions, and also on the electronic ground state of N2 by spontaneous Raman scattering, provide information of their vibrational distribution function. A spatial study has been carried out, showing a quite slow evolution of the vibrational populations along the short-lived afterglow, and in particular the first proof of a steady vibrational distribution of N2(X 1g+) from the beginning to the bulk of this ionized region. The N2(B 3g vibrational distribution is analysed by a steady-state kinetic model taking into account all the B state excitation processes, i.e. electron collisions and reactions between molecular excited states, and quenching. The reaction between the vibrationally excited N2(X 1g+) and N2(A3u+) is shown to be the pre-eminent excitation mechanism in the short-lived afterglow.

Detailed insight into the hydrogen bonding interactions in acetone–methanol mixtures. A molecular dynamics simulation and Voronoi polyhedra analysis study

Voronoi polyhedra (VP) analysis of mixtures of acetone and methanol is reported on the basis of molecular dynamics computer simulations, performed at 300 K and 1 bar. The composition of the systems investigated covers the entire range from neat acetone to neat methanol. Distribution of the volume, reciprocal volume and asphericity parameter of the VP as well as that of the area of the individual VP faces and of the radius of the empty voids located between the molecules are calculated. To investigate the tendency of the like molecules to self-associate the analyses are repeated by disregarding one of the two components. The self-aggregates of the disregarded component thus turn into large empty voids, which are easily detectable in VP analysis. The obtained results reveal that both molecules show self-association, but this behavior is considerably stronger among the acetone than among the methanol molecules. The strongest self-association of the acetone and methanol molecules is found in their mole fraction ranges of 02-0.5 and 0.5-0.6, respectively. The caging effect around the methanol molecules is found to be stronger than around acetones. Finally, the local environment of the acetone molecules turns out to be more spherical than that of the methanols, not only in the respective neat liquids, but also in their mixtures.

Thermodynamics of Mixing Water with Dimethyl Sulfoxide, as Seen from Computer Simulations

The Helmholtz free energy, energy, and entropy of mixing of eight different models of dimethyl sulfoxide (DMSO) with four widely used water models are calculated at 298 K over the entire composition range by means of thermodynamic integration along a suitably chosen thermodynamic path, and compared with experimental data. All 32 model combinations considered are able to reproduce the experimental values rather well, within RT (free energy and energy) and R (entropy) at any composition, and quite often the deviation from the experimental data is even smaller, being in the order of the uncertainty of the calculated free energy or energy, and entropy values of 0.1 kJ/mol and 0.1 J/(mol K), respectively. On the other hand, none of the model combinations considered can accurately reproduce all three experimental functions simultaneously. Furthermore, the fact that the entropy of mixing changes sign with increasing DMSO mole fraction is only reproduced by a handful of model pairs. Model combinations that (i) give the best reproduction of the experimental free energy, while still reasonably well reproducing the experimental energy and entropy of mixing, and (ii) that give the best reproduction of the experimental energy and entropy, while still reasonably well reproducing the experimental free energy of mixing, are identified.

Time and space-resolved analysis of perturbations caused by a flame propagating through, a gas mixture by C.W. Laser multichannel raman spectroscopy

Multichannel Raman spectroscopy using a continuous wave laser excitation allows measurements of temperature and compostion variatons of a gas mixture with a time resolution of 40 ms and the simultaneous probing of two or more spatial elements in the studied mixture. This technique is applied to the study of the thermal effect caused by a hydrogen-air flame propagating through a combustion chamber.

Méthode originale de détermination de la température dans une proche postdécharge d'azote

A quantitative study of spectra obtained by Raman-Stokes scattering enables the determination of the kinetic temperature of the N2(X1 Σg+) molecule in the short lived afterglow appearing downstream from a microwave discharge in flowing pure nitrogen. The obtained value agrees with the rotational temperature deduced from the emission spectra of the N2(B3 Πg)v′ = 2 species. The temperature deduced from the emissions of N2+(B2 Σu+)v′ = 0 shows a strong discrepancy with the former explained on the basis of the radiative lifetime of the species and of the time separating collisions of the molecular species.